\(\bullet \)  Strong coupling. Barring innovations, large(r) \(\lambda \) 
should not be the overriding aspiration. There are numerous cases, including compressed
hydrides (discussed above), where increasing \(\lambda \)  increases T\(_c\)  but rapidly
encounters soft phonons and lattice instability. \(\lambda \)  is a poor descriptor for
a high T\(_c\)  search. Clue: \(\eta /M\) , independent of frequencies, is much better.[1]}, [2]}
Since higher frequency scales will likely require ever higher pressures, the
alternative seems to be pushing strong coupling to the high frequency region,
which typically are the `bond stretch' modes.
